This invention relates generally to disk drive systems for storing digital data and more particularly to such systems utilizing servo information prerecorded on a dedicated disk surface for controlling tee positioning of read/write heads to read and/or write data on concentric tracks on other disk surfaces.
Typical high capacity disk drive units use a plurality of rigid magnetic disks stacked on a common spindle. For example only, such a unit may consist of five disks defining ten major disk surfaces with one surface being dedicated to storing servo information and the others being used for storing digital data. Such a disk drive unit typically includes a positioner subsystem, mounted adjacent to the disk stack, carrying a plurality of aligned read/write heads including at least one head per active disk surface (i.e. servo plus data surfaces). The heads are typically mounted on a common head arm which is coupled to a positioner motor. Energization of the positioner motor moves the arm to thus move each head radially relative to its disk surface. By properly controlling the positioner motor, the heads can seek, and then follow, any selected one of a plurality of concentric tracks prerecorded on the servo surface.
A disk drive unit is typically comprised of two primary portions; namely, a head disk assembly (HDA) and a controller electronics board. In the normal operation of such a unit, a track seek command is issued by a host computer to the controller electronic. The controller electronics determines the direction and magnitude of movement required to move the heads from their current track (or "cylinder") position to the new or destination track. Based on this information, the controller electronics selects an optimized velocity profile to rapidly move the head arm to position the heads over the destination track.
High performance disk drive unts attempt to maximize the density at which tracks are written. For example, it is not unusual for data tracks to be recorded at a density in excess of one thousand per radial inch. In order to achieve such high track densities and permit rapid head positioning, the positioner motor is usually operated in a closed servo loop. That is, as the positioner motor is energized to move the heads, the servo head (i.e. the head associated with the dedicated servo surface) counts track crossings of a prerecorded servo pattern until the head arrives at its destination track. For example, assume that the heads are currently at data track 19 and that the computer issues a command to move the heads to data track 739. In response, a track counter is set to the magnitude of movement required, i.e. 720, and the positioner motor is moved in accordance with a velocity profile which permits it to accelerate to a maximum velocity, maintain that maximum velocity for a certain interval, and then decelerate to reach zero velocity concurrent with the head arriving at the destination track. The velocity transition points are generally determined by the current count in the track counter which is decremented as the servo head detects track crossings of the servo pattern recorded on the dedicated servo surface.
The foregoing explanation is generally applicable to various state of the art disk drive systems well known in the technical literature and widely commercially available. One such system comprises the 1320 series of disk drive units marketed by Micropolis Corporation of Chatsworth, Calif. Other systems are described in many issued U.S. patents including:
______________________________________ 3,593,333 3,691,543 3,838,457 3,893,180 4,415,939 4,556,597 4,630,144 4,052,741 4,418,368 4,558,383 4,631,606 4,087,843 4,424,543 4,559,570 4,633,343 4,101,942 4,439,800 4,562,562 4,633,345 4,135,217 4,462,053 4,568,988 4,633,451 4,157,577 4,488,187 4,575,775 4,636,885 4,163,265 4,488,188 4,575,776 4,638,384 4,188,646 4,238,809 4,490,756 4,589,037 4,639,906 4,286,296 4,286,296 4,297,737 4,511,938 4,590,526 4,642,709 4,331,976 4,516,162 4,590,527 4,647,992 4,352,131 4,524,398 4,602,304 4,656,538 4,380,033 4,530,019 4,613,915 4,669,003 4,390,911 4,530,020 4,616,275 4,669,003 4,390,912 4,549,232 4,616,276 4,679,103 4,400,747 4,412,165 4,554,600 4,628,380 4,686,590 4,414,589 ______________________________________
The aforementioned Micropolis 1320 comprises an exemplary hard disk drive unit employing a dedicated surface on which position (or servo) information is recorded during the manufacturing process. In use, this position information is accessed by the servo head and processed by the units servo electronics to control the positioner motor. The recorded position information contains the following three types of information.
(1) Radial Position: The disk surface is partitioned into four major zones (concentric bands) as follows.
Outer guard band PA1 Data zone (track O of the data zone is specially coded) PA1 Inner guard band PA1 (Landing zone
(2) Rotational (circumferential) Position: A once per revolution index position is encoded.
(3) Position Error: Information is encoded to produce a two-phase position error signal (Reference and Quadrature). Each position error phase is a triangular waveform which varies cyclically in amplitude, plus or minus, as a function of the displacement of the servo head from the center of a data cylinder.
In the Micropolis 1320, as described in the Micropolis 1320 Maintenance Manual, Document 101420, Rev. B, October, 1985 the position information on the servo surface is organized into 1680 servo cells per track. Each servo cell contains a sync field and four (A, B, C, D) track centerline information fields. The cell sync field consists of two adjacent areas for respectively storing dipoles S1 and S2 (each dipole comprising a pair of opposite magnetic flux transitions). Dipole S1 is present in all servo cells and defines the cell boundary. Dipole S2 is selectively either present or absent and encodes the cell as a "1" or "0" cell. This cell encoding is used by the servo electronics to derive both rotational position and radial position. More specifically each servo track is divided into 30 pie-shaped sectors each containing 56 servo cells. In sector 0, each eight cell sequence (on all tacks) yields an eight bit word encoded to identify a disk rotational index position. In sectors 1-29, the cells are encoded to yield eight bit words identifying gross radial position in accordance with the following table:
______________________________________ Radial Position Radial Position Code ______________________________________ Guard Band 1 0 1 1 1 1 1 1 0 Guard Band 2 0 1 1 1 1 1 1 1 Track 0 1 1 1 1 1 1 1 0 Data Zone 1 1 1 1 1 1 1 1 ______________________________________
The track centerline information fields in each cell comprise areas in which A, B, C, D dipoles are either present or absent. These track centerline dipoles are arrange relative to the servo track boundaries to yield position error information enabling the servo head to follow a servo track centerline or boundary. The groups of A, B, C, D dipoles are offset with respect to servo track boundaries such that when the servo head is aligned with a track, it will read either a full or half or zero amplitude output for each dipole group in accordance with the following table:
______________________________________ Track Boundary A B C D (A - B) (C - D) ______________________________________ w 0.0 1.0 0.5 0.5 -1.0 0.0 x 0.5 0.5 0.0 1.0 0.0 -1.0 y 1.0 0.0 0.5 0.5 +1.0 0.0 z 0.5 0.5 1.0 0.0 0.0 +1.0 w 0.0 1.0 0.5 0.5 -1.0 0.0 ______________________________________
The foregoing is accomplished by an organization in which each servo cell, dependent on its position, includes in its four track centerline information fields either (1) an A and C or D dipole or (2) a B and C or D dipole.